1. Field of the Invention
The present invention relates to the architecture of completely optical high capacity crossconnected systems for multi-wavelength mode locked dense wavelength division multiplexed (MWML-DWDM) optical communication systems using multiwavelength mode locked (MWML) lasers as light sources for the transport of, e.g., data and telephony services in fiber optic networks.
2. Description of the Prior Art
There has been much research conducted over the years concerning the use of dense wave length division multiplexing (DWDM) systems to increase the bandwidth of existing and emerging fiber optic transmission systems. As shown in FIG. 1, a conventional approach has been to use a plurality of stabilized laser diodes 10 operating at or near 1550 nm with wavelength separations of 0.4, 0.8, or 1.6 nm (frequency separations of 50, 100, or 200 GHz) where each laser diode 10 is modulated at rates between 155 Mb/s and 2.5 Gb/s by wideband external modulators 12. These modulated optical carriers, or optical data streams, are combined together using an optical combiner 14 to construct a higher bit-rate optical data stream that can be amplified by one or more wideband power amplifiers 16 and can be inserted into one single-mode optical fiber 18.
Conventional DWDM optical transmission systems of the type illustrated in FIG. 1 can transmit data of very high bit rates over conventional fiber optic lines. As is apparent from FIG. 1, such conventional DWDM optical transmission systems require a plurality of light sources 10 and data modulators 12. These components are expensive and complex, and the resulting multicomponent systems are exceedingly complex and expensive. A less expensive alternative is desired.
Recently, multiwavelength mode-locked (MWML) laser light sources have been developed which generate a plurality of pulses of different wavelengths in which each wavelength emits picosecond pulses at high rates. When a conventional pulse interleaving configuration including delay line units is included within such MWML laser sources, the output optical pulse train may be multiplexed to provide very high pulse rates at each wavelength, such as those which are suitable for DWDM transmission. A MWML laser source of this type has been described by Shi et al. at a presentation entitled xe2x80x9cFour-Wavelength, 10 GHz Picosecond Pulse Generation From an Active Mode-Locked Single-Stripe Diode Laserxe2x80x9d given May 20, 1997, at the 1997 Conference on Lasers and Electro-Optics, and described in an associated paper entitled xe2x80x9c20xc3x975 Gbit/s Optical WDM Transmitter Using a Single-Stripe Multiwavelength Modelocked Semiconductor Laser.xe2x80x9d Commercial embodiments of such a MWML laser source are described in commonly assigned U.S. patent application Ser. No. 09/191,218 filed Nov. 12, 1998, now U.S. Pat. No. 6,192,058 issued Feb. 20, 2001. As described therein, a single mode-locked laser, preferably an actively mode-locked semiconductor external cavity laser (AMSECL), emits a multiplicity of fundamental optical frequency components. An RF drive signal is provided to a semiconductor optical amplifier (SOA) situated within an optical resonator such as a Fabry-Perot cavity or ring resonator by a clock source so that the SOA amplifies mode-locked pulses periodically. The SOA preferably comprises an angled-stripe InGaAsP or GaAs/AlGaAs semiconductor optical traveling wave amplifier with facet reflectivities of substantially 10xe2x88x926 or less. Such low reflectivity is necessary to keep the gain spectrum of the SOA free of undulations due to SOAFabry-Perot modes which otherwise would interfere with the generation of multiple wavelengths, since some wavelengths would be emphasized while others would be muted by such undulations. Each fundamental optical frequency component is associated with its own unique set of additional frequency components such that each fundamental optical frequency and additional frequency components make up a unique wavelength band (xe2x80x9ccombxe2x80x9d) of frequency components. As a result, a multiplicity of combs of optical frequency components is provided by the MWML laser source. In the time domain, the MWML laser source emits pulses of overall duration approximately equal to the inverse of the spectral width of each comb in the frequency domain.
Owing to the close wavelength spacing of the optical frequencies within a given comb, each comb may be considered as a single unique wavelength source for the purposes of DWDM systems.
In the aforementioned commonly assigned U.S. Patent Application, different embodiments of the MWML laser source are described, where each MWML laser source emits multiple discrete groups of wavelengths simultaneously in a short time interval, and each group is located at a wavelength suitable to DWDM optical transmission. Feedback or feedthrough optics modules are combined with gain modules to provide embodiments suitable for DWDM transmission. The optics are coupled to one or more ports of the amplifier in Fabry-Perot or optical ring resonator configurations to provide simultaneous feedback at the plurality of wavelengths and to provide substantially identical round-trip travel times and net gains within the lasing cavity for pulses at each output wavelength.
In accordance with the invention, it is desired to develop elements such as optical crossconnects for a DWDM optical transmission system so as to provide flexible, high capacity means for routing high bandwidth fiber optic data streams. It is particularly desired that the optical crossconnect allows fast, economical circuit switching capability using a MWML laser source of the type described in the aforementioned patent application. It is further desired in accordance with the present invention to develop add/dropmultiplexers and other components of a DWDM optical transmission system using such MWML laser light sources so as to provide an improved MWML-DWDM optical transmission system. The present invention has been designed to meet these needs in the art.
The present invention relates to system architecture and usage of a dense wavelength division multiplex (DWDM) crossconnect system and optical add/drop multiplexer which preferably, though not necessarily, uses a multi-wavelength mode locked (MWML) external cavity laser as a fundamental sub-circuit for optically cross-connecting several multi-wavelength data channels onto similar data channels having an interchanged set of optical wavelengths. The optical crossconnect architecture in accordance with the invention allows the rearrangement of multiple logical channels in a timely manner to support protection switching, traffic grooming, and other telecommunications network functions. The crossconnect architecture of the invention further supports multiple physical fibers into and out of the crossconnect, where each fiber carries multiple logical channels on multiple wavelength pulses. In accordance with the invention, any xe2x80x9clogical channelxe2x80x9d on a physical fiber that is assigned a particular wavelength carrier can be cross-connected onto any other physical fiber and assigned another wavelength carrier.
The present invention is generally directed to a network element for use in a dense wavelength division multiplexed (DWDM) optical network. In an add/drop configuration, the network element receives input optical signals from the DWDM optical network at a first plurality of input ports and input signals at a second plurality of input (ADD) ports for insertion into the optical network, and emits output optical signals to the DWDM optical network at a first plurality of output ports and output signals at a second plurality of output (DROP) ports.
In a preferred embodiment, the network element comprises at least one optical receiver that receives the input optical signals from the optical network at the first plurality of input ports and demodulates digital data from at least one designated wavelength of the input optical signals. The network element further comprises a multiwavelength optical source which emits wavelength ordered repetitive periodic (WORP) pulse sequences within a plurality of discrete wavelength bands at its output port, and at least one modulator which receives the WORP pulse sequences from the multiwavelength optical source and modulates the digital data from the at least one designated wavelength onto selected wavelengths of a WORP sequence to create the output optical signals for application to the DWDM optical network. Data streams to be added (via ADD ports) are multiplexed with the xe2x80x9cexpressxe2x80x9d (or through) data streams to comprise the signals to be modulated onto the output port optical carriers. Preferably, each output port contains a multiplicity of optical data channels each of which is assigned a unique wavelength from the WORP pulse sequences.
In the preferred embodiment, the multiwavelength optical source is a multiwavelength mode-locked (MWML) laser including a semiconductor optical amplifier which is actively driven by a radio frequency signal to emit periodic pulses at a plurality of wavelengths simultaneously at aport of the amplifier. The radio frequency signal is either a fundamental frequency or a harmonic of the radio frequency signal that substantially equals the inverse of a round trip travel time of pulses circulating within a cavity of the MWML laser. A temporal dispersion element time shifts the plurality of wavelengths by differential wavelength-dependent delays so as to transform pulses output by the MWML laser into a wavelength-sequential sequence for application to the modulator.
The network element of the invention may also be configured as a crossconnect with an optical fiber switch core for use in a dense wavelength division multiplexed (DWDM) optical network. Typically, the optical crossconnect receives a first plurality of electrical or optical data signals at input ports on an input side and provides a second plurality of electrical or optical data signals at output ports on an output side. In a preferred embodiment, the optical crossconnect comprises a switching control element which outputs a selection signal, a multiwavelength optical source which emits WORP pulse sequences within a plurality of discrete wavelength bands simultaneously, and at least one modulator which receives the WORP pulse sequences from the multi-wavelength optical source and modulates the first plurality of data signals from the input ports onto selected wavelengths of a WORP pulse sequence from the multiwavelength optical source so as to create a modulated WORP pulse sequence for transmission over the optical fiber switch core. The modulated WORP pulse sequence contains a multiplicity of optical data channels each of which is assigned a unique wavelength from the WORP pulse sequence. The modulated WORP pulse sequence is then received by least one optical receiver which demodulates digital data from wavelengths in the modulated WORP pulse sequence as selected by the selection signal and provides the digital data as the second plurality of data signals to the output ports.
Corresponding DWDM transmission methods are also described as will be apparent to one skilled in the art from the following description.